ACTG 362 is a randomized, double-blind, placebo-controlled trial which will be available for enrollment to any subject who is receiving antiretroviral therapy and has had a documented increase in CD4 cell count from below 50/mm on one occasion to above 100 cells/mm on two separate occasions, sequentially, at least 4 weeks apart (the last test being performed at an ACTG-approved laboratory within 14 days of study entry). There will be 636 subjects enrolled (318 in each arm). Subjects will be randomized to receive azithromycin 1200 mg once weekly or matching placebo and will be followed every 8 weeks until study closure or for 18 months (72 weeks) after the last is enrolled. Subjects who develop a drop in CD4 count below 50 cells/mm on two measurements at least 4 weeks apart will be offered open-label azithromycin. Subjects enrolled in ongoing antiretroviral therapy trials will be eligible for co-enrollment with permission from both study teams.